[FIX] l10n_it_stock_ddt: report language

Steps to reproduce:
- Set up an Italian language company with Italian localisation.
- Set up an English language customer.
- Set up an Italian language supplier.
- Set up a product with dropshipping options.
- Set up a sales order for the product to be dropshipped direct
 from the Italian supplier to the English customer.
- Print the DDT report.

Bug:
The report will be printed in Italian.

Fix:
same as delivery slip reuse _get_report_lang

opw-3526633

closes odoo/odoo#137638

X-original-commit: 10283d926f8e4d542ca0ae8b0d25710e650c9574
Signed-off-by: William Henrotin (whe) <whe@odoo.com>
Signed-off-by: Walid Hanniche (waha) <waha@odoo.com>

addons/l10n_it_stock_ddt/report/l10n_it_ddt_report.xml

@@ -178,7 +178,7 @@
     <template id="report_ddt">
         <t t-call="web.html_container">
             <t t-foreach="docs" t-as="o">
-                <t t-set="lang" t-value="o.partner_id.lang if o.partner_id.lang else o.env.lang"/>
+                <t t-set="lang" t-value="o._get_report_lang()"/>
                 <t t-call="l10n_it_stock_ddt.report_ddt_view" t-lang="lang"/>
             </t>
         </t>